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From: "Junio C Hamano via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: "Sibi Siddharthan" <sibisiddharthan.github@gmail.com>,
	"Đoàn Trần Công Danh" <congdanhqx@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	"Eric Sunshine" <sunshine@sunshineco.com>,
	"Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
	"Øystein Walle" <oystwa@gmail.com>,
	"Johannes Schindelin" <johannes.schindelin@gmx.de>,
	"Junio C Hamano" <gitster@pobox.com>
Subject: [PATCH v4 10/10] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3
Date: Wed, 30 Sep 2020 15:26:24 +0000	[thread overview]
Message-ID: <7643a766b7dedb32bbe46689b01a9009e16277f9.1601479585.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.738.v4.git.1601479585.gitgitgadget@gmail.com>

From: Junio C Hamano <gitster@pobox.com>

The OFFSETOF_VAR(var, member) macro is implemented in terms of
offsetof(typeof(*var), member) with compilers that know typeof(),
but its fallback implemenation compares &(var->member) and (var) and
count the distance in bytes, i.e.

    ((uintptr_t)&(var)->member - (uintptr_t)(var))

MSVC's runtime check, when fed an uninitialized 'var', flags this as
a use of an uninitialized variable (and that is legit---uninitialized
contents of 'var' is subtracted) in a debug build.

After auditing all 6 uses of OFFSETOF_VAR(), 1 of them does feed a
potentially uninitialized 'var' to the macro in the beginning of the
for() loop:

    #define hashmap_for_each_entry(map, iter, var, member) \
            for (var = hashmap_iter_first_entry_offset(map, iter, \
                                                    OFFSETOF_VAR(var, member)); \
                    var; \
                    var = hashmap_iter_next_entry_offset(iter, \
                                                    OFFSETOF_VAR(var, member)))

We can work around this by making sure that var has _some_ value
when OFFSETOF_VAR() is called.  Strictly speaking, it invites
undefined behaviour to use NULL here if we end up with pointer
comparison, but MSVC runtime seems to be happy with it, and most
other systems have typeof() and don't even need pointer comparison
fallback code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 hashmap.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hashmap.h b/hashmap.h
index ef220de4c6..b011b394fe 100644
--- a/hashmap.h
+++ b/hashmap.h
@@ -449,7 +449,8 @@ static inline struct hashmap_entry *hashmap_iter_first(struct hashmap *map,
  * containing a @member which is a "struct hashmap_entry"
  */
 #define hashmap_for_each_entry(map, iter, var, member) \
-	for (var = hashmap_iter_first_entry_offset(map, iter, \
+	for (var = NULL, /* for systems without typeof */ \
+	     var = hashmap_iter_first_entry_offset(map, iter, \
 						OFFSETOF_VAR(var, member)); \
 		var; \
 		var = hashmap_iter_next_entry_offset(iter, \
-- 
gitgitgadget

      parent reply	other threads:[~2020-09-30 15:26 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-25 14:28 [PATCH 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-25 16:25   ` Sibi Siddharthan
2020-09-26 20:32     ` Johannes Schindelin
2020-09-27  2:25       ` Đoàn Trần Công Danh
2020-09-28 13:56         ` Johannes Schindelin
2020-09-29 14:04           ` Đoàn Trần Công Danh
2020-09-29 18:42             ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-25 14:28 ` [PATCH 06/10] cmake (Windows): let the `.dll` files are found when running the tests Johannes Schindelin via GitGitGadget
2020-09-25 19:48   ` Eric Sunshine
2020-09-26 21:00     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-25 17:29   ` Sibi Siddharthan
2020-09-26 20:33     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-30  5:05   ` Sibi Siddharthan
2020-09-30 15:25     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-25 18:22   ` Junio C Hamano
2020-09-26 20:45     ` Johannes Schindelin
2020-09-25 14:28 ` [PATCH 10/10] hashmap_for_each_entry(): work around MSVC's run-time check failure #3 Johannes Schindelin via GitGitGadget
2020-09-25 18:38   ` Junio C Hamano
2020-09-26 16:54     ` Junio C Hamano
2020-09-26 20:57       ` Johannes Schindelin
2020-09-26 21:32 ` [PATCH v2 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-28 11:17     ` Øystein Walle
2020-09-28 19:39       ` Johannes Schindelin
2020-09-26 21:32   ` [PATCH v2 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 06/10] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-26 21:32   ` [PATCH v2 10/10] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3 Junio C Hamano via GitGitGadget
2020-09-28 21:09   ` [PATCH v3 00/11] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 01/11] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 02/11] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 03/11] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 04/11] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 05/11] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 06/11] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 07/11] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 08/11] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-29  6:51       ` Sibi Siddharthan
2020-09-29 12:07         ` Johannes Schindelin
2020-09-28 21:09     ` [PATCH v3 09/11] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 10/11] hashmap_for_each_entry(): workaround MSVC's runtime check failure #3 Junio C Hamano via GitGitGadget
2020-09-28 21:09     ` [PATCH v3 11/11] cmake: fix typo in message when `msgfmt` was not found Johannes Schindelin via GitGitGadget
2020-09-28 22:11       ` Junio C Hamano
2020-09-29 12:07         ` Johannes Schindelin
2020-09-30 15:26     ` [PATCH v4 00/10] CMake and Visual Studio Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 01/10] cmake: ignore files generated by CMake as run in " Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 02/10] cmake: do find Git for Windows' shell interpreter Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 03/10] cmake: ensure that the `vcpkg` packages are found on Windows Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 04/10] cmake: fall back to using `vcpkg`'s `msgfmt.exe` " Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 05/10] cmake: quote the path accurately when editing `test-lib.sh` Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 06/10] cmake (Windows): let the `.dll` files be found when running the tests Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 07/10] cmake (Windows): complain when encountering an unknown compiler Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` [PATCH v4 08/10] cmake (Windows): initialize vcpkg/build dependencies automatically Johannes Schindelin via GitGitGadget
2020-09-30 19:17         ` Johannes Schindelin
2020-09-30 23:08           ` Junio C Hamano
2020-09-30 15:26       ` [PATCH v4 09/10] cmake (Windows): recommend using Visual Studio's built-in CMake support Johannes Schindelin via GitGitGadget
2020-09-30 15:26       ` Junio C Hamano via GitGitGadget [this message]

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